Valves for such applications can be broadly divided into two categories, namely active and passive. Active valves have an external means of actuation, while passive valves are activated only by pressure changes occurring during normal operation of the system.
Conventional diesel engines use active valve assemblies in which, typically, a cam and spring arrangement open and close poppet valves. This is a simple mechanical device which will always operate at the same point in the cycle for all engine speeds. In such active valve assemblies it is known, for example, from a U.S. Pat. No. 5,553,572 to replace a conventional mechanical spring with an air spring. This takes the form of the end of the poppet valve remote from the head having an air filled chamber. On opening the valve, the air is compressed by the cam shaft which opens the valve. This compressed air is then used to provide the closing force for the valve.
More recently, a drive towards optimising engines for different operating conditions has led to other types of active valve actuator, such as hydraulic, pneumatic and electromagnetic actuators, being considered. These allow the timing of the valve motion to be varied while the engine is running. The principle of compressing air which is used to do useful work is also used in pneumatically operated valves such as those disclosed in U.S. Pat. Nos. 5,022,359, 5,152,260, 5,259,345 and EP-A-0,554,923. In all of these cases pneumatic pressure is applied to one side of a piston forming part of the valve assembly. The piston then moves to open the valve assembly and compress air, the pressure of the compressed air being used to return the valve to its original position. The pressure within the assembly is controlled such that the valve element is held in certain positions in order to provide the necessary open time for the valve to satisfy the engine requirements. Control of the movement of the piston is achieved by selectively introducing high pressure air into the chambers on either side of the piston and/or venting air from these chambers. These devices consume a significant amount of compressed air which is vented out of the chambers, thereby wasting energy.
Passive valves such as plate valves are generally found in conventional or reciprocating air compressors. These operate passively in response to the changing pressure in the cylinder and close when the pressure drop due to the flow drops below a certain level. No external control, be it mechanical, hydraulic, pneumatic or electromagnetic is applied to influence the valve element during a single stroke.
Such active and passive valves are widely used with great success in many applications. However no prior art design has been found to be successful for an application such as reciprocating compression or expansion with a high pressure ratio, which requires the valve to be open for a relatively short duration and occupy a small volume of the cylinder without causing high pressure losses or high parasitic power consumption. A cycle having components which require such characteristics is that disclosed in WO 94/12785. This document discloses a combined reciprocating isothermal compression and internal combustion cycle. It has been found that the discharge valve on the compressor is required to be open for only about 40° of crank angle. This compares with a conventional diesel engine in which the discharge valve is open for about 150° of crank angle.
Conventional active actuators are not capable of operating valves at the size required for such an application and at the desired speed without having a high parasitic power consumption.
Conventional passive actuators are also unacceptable. Plate valves need to be large in diameter for a given flow. This is not a problem if the compression ratio is low as it is in a conventional reciprocating air compressor, since there is sufficient volume at the top of the cylinder when the valves are open. However, if a high compression ratio is sought then the volume available when the valves open is small.